The present invention relates to a method of discriminating a type of a fuel to be supplied to an engine of a motor vehicle. More particularly, the present invention relates to a method of discriminating an octane number of a fuel. In other words, in the present invention, determination is made as to whether the gasoline has a high-octane number or a low-octane number.
It is well known to control an ignition timing at which an ignition plug sparks in accordance with an engine rotational speed and an engine load as well as an occurrence of a knocking of the engine. In this case, when the knocking is detected the ignition timing is retarded or approached the side of the top dead center of the crank shaft of the engine.
You can access two types of gasolines, i.e., the high-octane gasoline and the low octane gasoline. In this specification, the term of the low-octane gasoline means that an octane number of the low-octane gasoline is less than that of the high-octane gasoline. On the other hand, two types of gasoline engines optimum for the high-octane gasoline and the low-octane gasoline, respectively, are available for the consumer. It is well known that a maximum performance of the engine is achieved when the ignition advance angle is advanced to a critical ignition advance angle relating to the knocking of the engine. In case the ignition advance angle is advanced beyond the critical ignition advance angle, the engine would knock.
It is also well known that the more the octane number of the gasoline gets, the more the critical ignition advance angle can get.
In this connection, the following problems may be raised. In case the low-octane gasoline is supplied to the gasoline engine (referred to as "an engine for high octane gasoline" hereinafter) designed suitably for the high-octane gasoline, the knocking of the engine would frequently occur since an ignition plug may be ignited at the ignition advance angle more than an optimum or critical ignition advance angle for the low-octane gasoline. In case the high-octane gasoline is supplied to the gasoline engine (referred to as "an engine for low-octane gasoline" hereinafter) designed suitably for the low-octane gasoline, a maximum performance of the engine, which is achieved when the low-octane gasoline is used, may not be expected to be obtainable. This is because the ignition plug may be ignited at the ignition advance angle which is retarded too much with respect to the critical ignition advance angle. More specifically, by supplying the high-octane gasoline to the engine for low-octane gasoline, the ignition advance angle could be advanced to the critical ignition advance angle for the high-octane gasoline so that an output torque of the gasoline engine increases and an exhaust gas temperature decreases resulting in decrease of a fuel injection rate. However, since the gasoline engine for low-octane gasoline provides the ignition advance angle optimum for low-octane gasoline, no critical ignition advance angle for high-octane gasoline is obtainable. This causes the above-mentioned disadvantage.
In order to obviate these disadvantages, there has been proposed in Japanese Patent Application laid-open No. 57072/1983 an apparatus for controlling an ignition timing in which two tables of ignition timings for high and low octane gasoline are prestored and one of the tables is selected by a driver's switching operation of a switch for selecting of the table. Such a switch may be provided on an instrument panel of the motor vehicle. This apparatus, however, is subjected to the following problem. An erroneous switching operation of the switch for selection of the above table causes the same advantages mentioned above. This is because the driver has to determine which one of high and low-octane gasolines is supplied to the engine so that one of the tables of the ignition timing is selected by switching operation of the switch.
Further, there has been proposed in Japanese Patent Application laid-open No. 231980/83 an apparatus for controlling an ignition timing in which determination as to the type of the gasoline to be supplied to the engine is carried out in accordance with a knocking condition of the engine. However, the proposed apparatus necessitates an additional ignition advance angle map for discrimination of the type of the gasoline to be supplied together with a well-known ignition advance angle map for general control of the ignition timing based on an engine operational condition. In discriminating the type of the gasoline to be supplied, the ignition advance angle is selected from the additional ignition advance angle map, at first, and then the ignition plug sparks at the ignition advance angle thus selected. Thereafter determination is made as to whether the knocking of the engine takes place or not. If it is determined that the knocking of the engine takes place, it is found that an undesirable gasoline, e.g., low-octane gasoline is used. Such sequential procedures of discrimination of the gasoline requires a relatively longer period of time. In addition, in case no additional ignition advance angle map is provided, the ignition advance angle for discrimination of the type of the gasoline has to be calculated from the ignition advance angle for general control of the ignition timing based on the engine operational condition.
Furthermore, it is well known that the engine tends to knock under a rapid acceleration of the engine regardless of the types of the gasoline used. If the detection of the knocking of the engine due to the rapid acceleration is used in order that discrimination of the type of the gasoline is made, such a discrimination may become inaccurate.